Plastic optical lenses were introduced as alternatives to glass lenses having problems such as a high specific gravity and low impact resistance. As representative examples, there are polyethylene glycol bis(allyl carbonate), polymethylmethacrylate, diallyl phthalate, and the like. Although optical lenses prepared from the polymers exhibit superior properties such as moldability, dyeability, adhesive properties of hard coated films, impact resistance, and the like, lenses are thickened due to a low refractive index of approximately 1.50 (nD) and 1.55 (nD). Accordingly, in order to reduce the thicknesses of lenses, a variety of attempts have been made to develop an optical material having a high refractive index.
Korean Application Pub. Nos. 1993-0006918 and 1992-0005708 suggest a thiourethane based lens in which a polythiol compound and a polyisocyanate compound are reacted. In addition, Korean Patent No. 10-0681218 suggests a polythioepoxy based plastic lens. The thiourethane based lens, one of existing lenses, has advantages such as a high refractive index and superior impact strength. However, the thiourethane based lens has drawbacks such as a soft surface, a dented center, and the like. In addition, the thiourethane based lens has a problem such as rapidly reduced Abbe's number with increasing refractive index. The polythioepoxy based lens has properties such as a high refractive index and a high Abbe's number. However, the polythioepoxy based lens is fragile and dying thereof is not easy. To resolve the problems, a method of copolymerizing two resin types having different properties, namely, a method of copolymerizing a polythioepoxy compound, a polythiol compound, and a polyisocyanate compound, was suggested in Korean Patent No. 10-0417985, Japanese Application Pub. No. hei 11-352302, and the like.
However, in the cases of the polythioepoxy based lens, or the lens in which polythioepoxy and thiourethane are copolymerized, an undesired coloring phenomenon reducing transparency and thermal strain due to exposure to high temperature may be exhibited. Since such a coloring phenomenon and thermal strain deteriorate lens quality, there is a need for quality improvement.